For really cold weather it doesn't really matter much if the material is waterproof or not, and if they keep your feet warm at -25 that pretty much insures that the boots will be too hot when it's slushy out, especially if they're knee-high. Maybe it would be easier (but not cheaper ) if you look for a pair of warmish rubber boots for slushy weather (or just ones with room for thick wool socks in), and plain warm snow boots for the cold, that needn't be waterproof?

I usually wear roomy hiking boots in winter, sometimes plus gaiters, when I'm not wearing my pig farmer knee high rubber boots. I do have a pair of superwarm Sorel Glacier boots, bought for work, never worn...